Функция mb_substr_replace

if (!function_exists("mb_substr_replace")) 
{
    function mb_substr_replace($string, $replacement, $start, $length=null, $encoding=null) {
        if ($encoding == null) $encoding = mb_internal_encoding();
        if ($length == null) {
            return mb_substr($string, 0, $start, $encoding) . $replacement;
        }
        else {
            if($length < 0) $length = mb_strlen($string, $encoding) - $start + $length;
            return 
                mb_substr($string, 0, $start, $encoding) .
                $replacement .
                mb_substr($string, $start + $length, mb_strlen($string, $encoding), $encoding);
        }
    }
}